In curing tobacco by the procedure generally referred to as the "bulk curing" method, tobacco leaves are loaded in a relatively compact mass in racks or in containers and placed inside of an enclosed curing barn where a furnace circulates a forced flow of heated air through the mass of tobacco leaves to effect curing and drying.
In the first stage of the curing process, known as the "yellowing" stage, the tobacco is heated to temperatures on the order of 100.degree. F. under relatively high humidity conditions to promote certain chemical reactions in the tobacco which cause the green leaves to turn yellow. Once the tobacco is sufficiently yellowed, the relative humidity in the barn is reduced and the temperature of the air is increased. These conditions set or fix the color in the leaf and promote drying of the tobacco leaves--first of the relatively thin leafy portion of the leaves, and later drying of the relatively thicker stem portions.
In each stage of the curing process, the temperature and humidity conditions within the barn must be carefully controlled. Departure from the proper curing conditions may result in inferior quality of the cured tobacco or even in total loss of the tobacco in the barn.
To insure that the proper curing conditions are maintained, it is conventional procedure to follow a preestablished schedule of temperature and humidity conditions during the various stages of curing.
By way of example, a typical curing schedule for use in a bulk curing barn might call for maintaining a 100.degree. F. temperature in the barn after initially loading the barn with tobacco and until yellowing is completed. During this time little or no fresh air is introduced into the barn so that the humidity is maintained at a relatively high level of about 85 to 95 percent relative humidity. After yellowing is accomplished to a sufficient degree, the temperature is then advanced to 2.degree. F. per hour up to 130.degree. F., and then maintaining 130.degree. F. until all of the leafy portions of the tobacco leaves are dry. During this time fresh air is introduced into the barn to reduce the relative humidity and promote drying. Then the temperature is again advanced at 2.degree. F. per hour up to 160.degree. F. to accomplish drying or killing of the stems. Typically this curing schedule might take about six days.
Control over the temperature during each stage of the cure is conventionally done with either manually set thermostats, or with the use of automatic temperature advance thermostats. For examples of the use of automatic temperature advance devices, reference may be made to the Flegel U.S. Pat. No. 3,203,265 issued Aug. 31, 1965 and Wilson U.S. Pat. No. 3,503,137 issued Mar. 31, 1970.
The use of a pre-established curing temperature schedule is at best an arbitrary approximation of the time and temperatures required for curing tobacco grown under average conditions. Often such a schedule does not provide the optimum curing conditions needed for the characteristics of a particular cropping of leaf. It is well known for example that the characteristics of tobacco leaves vary depending upon the location of the leaf on the tobacco plant, as well as upon growing conditions. Tobacco grown in relatively wet weather is quite different from tobacco grown in drier weather, releasing its moisture at a different rate, and consequently requiring different curing conditions.